1. Field of the Invention
The present invention relates to a structure of a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element such as an insulated-gate type transistor, and to a method of manufacturing the same. In particular, the present invention relates to a structure of a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element having the LDD structure formed with the use of a resist, and a method of manufacturing the same. A semiconductor device according to the present invention includes not only an element such as a thin film transistor (TFT) or a MOS transistor but also an electro-optical device such as a display device or an image sensor which has a semiconductor circuit consisting of the insulated-gate type transistor. In addition, a semiconductor device of the present invention also includes an electronic equipment provided with those electro-optical devices.
2. Description of the Related Art
What have attracted attention is an active matrix type liquid crystal display device in which a pixel matrix circuit and a driver circuit consist of a thin film transistor formed on an insulating substrate. A liquid crystal display of about 0.5 to 20 inches in size is utilized as a display device.
A TFT having as an active layer a crystalline semiconductor film, typical example of which is a polysilicon film, at present receives attention in an attempt to realize a liquid crystal display capable of displaying with high definition. However, the TFT having as the active layer the crystalline semiconductor film is, on one hand, higher in the operation speed and driving performance as compared with a TFT having as an active layer an amorphous semiconductor film, but, on the other hand, has a problem of significant fluctuation in characteristics between one TFT and another.
As one of the factors in this fluctuation in TFT characteristics, the interface between the active layer and the gate insulating film may be named. When contaminated, this interface affects the TFT characteristics. It is therefore important to purify the interface between an active layer and an insulating film that is in contact with the active layer.
Sought in a TFT now is high mobility, and the potential for use as an active layer of a TFT is a crystalline silicon film that has higher mobility than an amorphous silicon film. An outline of a conventional method of manufacturing a TFT will be described below in a simplified manner.
First, a gate wiring is formed on an insulating substrate, and a gate insulating film and an amorphous silicon film are layered thereon to form a polysilicon film by applying on this amorphous silicon film a crystallizing process such as heating or laser light irradiation. Subsequently, this polysilicon film is patterned into a desired shape to form an active layer. The polysilicon film is then selectively added with impurities that give P type or N type conductivity to form an impurity region to be a source region or a drain region. An interlayer insulating film is subsequently formed by deposition, a contact hole is formed to expose the source region or the drain region and, thereafter, a metal film is formed and patterned to form a metal wiring that is brought into contact with the source region or the drain region. The manufacturing process of a TFT is thus completed.
As is described, conventionally, an amorphous semiconductor film is exposed to the air because an insulating film is not formed until several steps (a crystallizing step and a patterning step, for example) are applied after formation of the amorphous semiconductor film.
The air within a clean room, in particular, contains mainly boron escaped from an HEPA filter that is commonly used for purifying, and boron is mixed in the active layer in an uncertain amount when the active layer is exposed to the air. In a conventional case where the active layer is fabricated being exposed to the air, the SIMS analyzation shows that the concentration of boron peaks (the concentration peak is shown by the dotted line B in FIG. 15) at the interface (on the main front surface side or on the back surface side) of the active layer of a TFT, and its maximum value reaches 1.times.10.sup.18 atoms/cm.sup.3 or more. When boron is mixed in the active layer as above, the concentration of impurities in the active layer is difficult to control and the mixed boron causes the threshold of the TFT to vary. To manufacture the layer with the use of other filter costs, and hence is not a proper solution for the problem.
As described above, conventionally, a semiconductor film is formed only to expose its surface to the air, thereby contaminating itself, i.e. the semiconductor layer to be an active layer, with impurities in the air (boron, oxygen, moisture, sodium, etc.). Otherwise, a gate insulating film is formed and then contaminated from the exposure to the air and, in turn, it contaminates with impurities in the air (boron, oxygen, moisture, sodium, etc.) a semiconductor film that is formed to be an active layer on the gate insulating film. When a semiconductor element, for example, a TFT is manufactured using thus contaminated semiconductor film, characteristics of the interface between the active layer, in particular, a channel formation region and the gate insulating film are degraded, causing fluctuation and degradation in electrical characteristics of the TFT. Also, the impurities (boron, oxygen, moisture, sodium, etc.,) inhibit crystallization of the semiconductor film at the crystallization step.